Following the natural progression from monaural sound to the universal acceptance of stereo sound in many aspects of professional and entertainment audio, surround sound is furthering this progression in an emerging stage of development.
Stereo sound generally provides the listener with the perception of a sound stage extending between and somewhat beyond the angle of the two loudspeakers, typically located at angles+/−30 degrees from the listener's forward axis. Human binaural hearing is able to perceive the direction of a sound source by comparing the arrival time of the sound at each ear, due to the separation of the listener's ears causing a geometric difference in the length of the off-axis sound path and by interpreting the difference in sound level presented to each ear. In sound reproduction the sound direction using only a pair of loudspeakers is controlled by panning, or controlling the volume of each loudspeaker, either by using directional microphones during recording or by manipulating the process after recording.
While in most stereo listening experiences there is little or no perception of the acoustic space, in the better recordings there can be a sense of a sound stage in front of the listener, where a sensation of depth can be achieved. The creation of a surround sound environment using loudspeakers usually requires additional loudspeakers around the listener to fill in the sounds coming from the side and rear. This has lead to the commonly used formats designated 5.1, 5.2, 7.2, etc. in movie and home theater applications. These all-loudspeaker systems seek to not only expand the angle of the sound stage from about 60 degrees to full 360 degrees, but also to provide the listener with greater perception of source distance, and thus a more realistic perception of the acoustic space.
Binaural perception of the distance to a sound source is highly dynamic and complex, involving many subtle and/or subjective factors that vary with the source distance, e.g., small inter-channel/binaural differences in loudness, deviations from expected familiar patterns of harmonic content in music and in speech due to the loss of high frequency information along with attenuation as a function of distance, and reverberant field contributions due to room acoustics (both in origination and in reproduction) involving direct-to-reverberant ratios controlled by the room.
There have also been many attempts to reproduce a three dimensional sound field through the use of binaural or dummy head recording along with headphone playback. These generally require careful attention to the recording chain and are sensitive to breaks in the process. The primary difficulty encountered with this technique is that sounds that originate from in front of the head are perceived by the listener as originating inside his head. Thus frontal sounds are poorly localized. Clearly the two reproduction techniques, loudspeakers and headphones each have an area where they are preferred: loudspeakers for front sounds and headphones for side and rear sounds.